Thermodynamic properties of the ionization of morpholine as a function of temperature and ionic strength

A hydrogen-electrode concentration cell was used to monitor pH in a study o f morpholine ionization in aqueous solutions to temperatures of 290 degrees C and to ionic strengths of 1 m, maintained with either sodium trifluorome thanesulfonate or sodium chloride. The resulting hydrolysis quotients were combined with values from two previous potentiometric investigations of thi s reaction, as well as existing heat capacity and apparent molar volume dat a, to provide a global fitting equation. The computed thermodynamic paramet ers for the acid dissociation of the morpholinium ion at 25 degrees C and i nfinite dilution are log K = (-8.491 +/- 0.003); Delta H = (39 +/- 1) kJ.mo l(-1); Delta S = (-31 +/- 1) J.K-1 mol(-1); Delta C-p = (40 +/- 7) J.K-1.mo l(-1); and Delta V = (3 +/- 1) cm(3).mol(-1). At low ionic strengths, inclu ding values at infinite dilution, the log Q data were found to be near line ar with respect to the reciprocal temperature in Kelvin over the measured t emperature range of 0 degrees C to 300 degrees C.